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Imaging Moving Wet Stuff: An Introduction to Seismic Oceanography

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Sedgwick Club Conference 2015

Over the last 50 years, controlled source seismic reflection imaging of the sub-surface has made spectacular strides. In the 1960s, you were doing well if you could make ghostly images of anticlines. Now, we can map buried landscapes in three dimensions and watch injected carbon dioxide flow through saline aquifers. A few years ago, it was realised that this technique can also be used to image the detailed structure of thermohaline circulation within the oceanic realm. It turns out that typical seismic reflection experiments are ideally suited to image what physical oceanographers confusingly call fine structure (i.e. length scales of 10s to 1000s of metres). However, it is crucial that the seismic records are first carefully reprocessed since acoustic impedance changes within the water column are one or two orders of magnitude smaller that those encountered in the solid Earth. The resulting two-dimensional images are spectacular vertical slices through the water column down to abyssal depths. They have vertical and horizontal resolutions of 5—10 meters and we can image temperature changes as small as 0.03 degrees Centigrade. Thanks to hydrographic calibration, we are confident that our images are real but we see a variety of puzzling features which do not have straightforward physical oceanographic explanations. The most significant issue that can be tackled concerns diapycnal (i.e. across density boundaries) mixing. Recent work has shown that spectral analysis of seismic images reveals important details about the nature of this mixing process.

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